| ↑ 2 blocks not present in 1869 1872; present in 1859 1860 1861 1866 |
| Let him
remember Lyell's profound remark,
that the thickness and extent of sedimentary formations are the result and measure of the degradation which the earth's crust has elsewhere suffered.
And what an amount of degradation is implied by the sedimentary deposits of many countries! |
|
| → .. .. .. .. .. .. 1869 |
| OMIT 1872 |
|
| → .. .. .. .. .. .. 1869 |
| OMIT 1872 |
|
| → periods. 1859 1860 1861 1866 1869 |
| periods of enormous length. 1872 |
|
| ↑ 1 blocks not present in 1869; present in 1859 1860 1861 1866 1872 |
| Good observers have estimated that sediment is deposited by the great Mississippi river at the rate of only 600 feet in a hundred thousand years.
|
|
| → rock, disintegrated through subaerial agencies, 1869 |
| solid rock, as it became gradually disintegrated, 1872 |
|
|
are good to show how slowly the mass must have been heaped together. ↑ Professor Ramsay has given me the maximum thickness, from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:—
Palæozoic strata (not including igneous beds) ..
57,154
Secondary strata ..
→.. .. .. .. .. .. 13,190
Tertiary strata ..
→.. .. .. .. .. .. 2,240 — making altogether 72,584 feet; that is, very nearly thirteen and three-quarters British miles. Some of
formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent. Moreover, between each successive formation, we have, in the opinion of most geologists,
blank
→periods. So that the lofty pile of sedimentary rocks in
gives but an inadequate idea of the time which has elapsed during their
The consideration of these various facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity. ↑
|
|
| Nevertheless this impression is partly false. Mr. Croll, in
interesting paper, remarks that we do not err "in forming too great a conception of the length of geological periods," but in estimating them by years. When geologists look at large and complicated phenomena, and then at the figures representing several million years, the two produce a totally different effect on the mind, and the figures are at once pronounced
too small.
regard to
Mr. Croll shows, by calculating the known amount of sediment annually brought down by certain rivers, relatively to
areas of drainage, that 1000 feet of
→rock, disintegrated through subaerial agencies,
would thus be removed from the mean level of the whole area in the
|
